In constructing hybrid microcircuits, metal layers are formed on ceramic substrates from metal powders which are bonded to the substrate by utilizing glazing paste. The metal powders can be mixtures of the noble metals such as gold, silver, platinum and palladium and the glazing paste is glass frit suspended in an organic vehicle. The metal powder combined with the glazing paste are applied to the substrate and heated to about 900.degree. C. to remove the organic vehicle, to melt the glass frit which fuses the metal to the substrate and to alloy the metal powder constituents.
In this process, however, the glass frit causes a glass surface to form on the formed metal surface which is referred to as overglazing. The overglazing presents a problem when soldering onto the metal surface is required, since solder does not adhere well to the overglaze. The surface overglaze can be removed by abrasion or burnishing, but this is an expensive, labor consuming process, and often weakens the bond between the metal and the ceramic substrate. Additionally, damage may result to adjacent circuit components.
In U.S. Pat. No. 3,903,344 to Laiming, a solution to this problem is described. A two layer glazing approach is employed wherein a first, lower layer rich in glass frit is fired onto the ceramic substrate and then a second, upper layer, having a low glass frit content but a high metal content, is fired onto the lower, glass-rich layer. When the first glass-rich layer is sufficiently thin and the second metal rich layer is sufficiently thick, no overglazing occurs and solder will adhere well to the metal layer.
It has been desired, however, to use a comparatively thicker, glass-rich first layer with a thinner, metal-rich second layer and still avoid overglazing. A thicker, glass-rich layer provides for strong bonding and a thinner, metal-rich layer reduces cost.